1. Field of the Invention
The invention is a single layer multi-color luminescent display and method of making and more particularly to thin-film electroluminescent displays.
Thin-film, multi-color electroluminescent (TFEL) flat-panel displays, because of their potential to provide improved flexibility and reliability, reduce weight, space, power consumption and degration characteristics, are finding greater use in the control panels of air, space and ground vehicles and many other applications requiring thin, flat, mutli-colored displays.
2. Description of the Prior Art
Full-colored electroluminescent displays formed of patterned and stacked layers of phosphors separated by insulating layers and transparent conductors and frequently filters are generally known. For instance, see U.S. Pat. No. 4,689,522, dated Aug. 25, 1987 which discloses a full-color, thin-film electroluminescent device with two stacked substrates and color filters. Multi-color electroluminescent displays formed by depositing side-by-side stripes of different colored phosphors on a common insulator substrate are also known
Conventional electroluminescent (EL) displays are generally divided into two major types according to the manner or form in which the phosphors are applied to the necessary substrate. These are thin-film electroluminescent (TFEL) and powder electroluminescent (powder EL) devices.
Powder EL devices are formed by grinding the phosphor crystals to be used into a powder, mixing the powder with a binder and a solvent, and then spreading the mixture (single color) onto a substrate by spraying or blading. TFEL devices are formed by growing the phosphors (single color) on a substrate using conventional techniques such as vapor deposition or sputtering.
Typically, the thickness of the phosphors layer in EL devices is about 20 to 40 .mu.M while the thickness of the phosphors layer in a TFEL device is 0.4 to 0.5 .mu.M. As is known the luminescence in a TFEL device is produced by a different mechanism in a powder EL device.
To display the full color spectrum including white, a conventional TFEL device will typically have the three primary and separate colors, blue, green and red phosphors, placed close together either side-by-side on the same substrate; on separate superimposed layers, or in some combination of these two fabrication techniques.
Typically, the three phosphors are applied to the substrate or substrates (in thickness of 2000 to 5000 Angstroms) by vacuum deposition. In conventional single layer TFEL devices alternating stripes of blue, green and red phosphors are grown on a glass substrate. In a two-layer TFEL device such as disclosed in U.S. Pat. No. 4,689,522, a single layer of blue phosphor is superimposed over a single layer of side-by-side alternating stripes of green and red phosphors.
The fabrication of a conventional multi-color TFEL device is generally as follows: After depositing a pattern of transparent electrodes on the surface of a glass substrate and covering it with a transparent layer of insulation, the following steps are performed: (1) a red phosphor is deposited as previously described over the insulated surface of the substrate: (2) the phosphor coated surface is masked with a striped pattern (commonly with photo-resist); (3) plasma etching of the red phosphor; (4) removal of the photo-resist; (5) deposition of a green phosphor; (6) the addition of an insulating layer; (7) the repetition of steps (2), (3), and (4), after the deposition of each additional colored phosphor; and (8) annealing of the phosphors. Variations in this process may be made by changing the order and repetition of the above steps or by ion-beam etching instead of plasma etching.
As is apparent, a disadvantage of the prior art is the necessity of the etching steps, the depths and locations of which must be precisely controlled. For instance, in the first etching step, the etching must continue through the full depth of the red phosphor layer but must be stopped before going into the insulating layer. In the second etching step, the etching must continue through the full depth of the green phosphor but stop before entering the red phosphor layer. The etching also leaves an uneven surface on the underlying phosphor layer that is believed to promote dielectric breakdown in the covering insulating layer applied after etching is completed.